Re: [PATCH 09/10] mm/hmm: allow to mirror vma of a file on a DAX backed filesystem

[Dan Williams]

On Wed, Jan 30, 2019 at 10:36 AM Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
[..]
> > > This
> > > is one of the motivation behind HMM ie have it as an impedence layer
> > > between mm and device drivers so that mm folks do not have to under-
> > > stand every single device driver but only have to understand the
> > > contract HMM has with all device driver that uses it.
> >
> > This gets to heart of my critique of the approach taken with HMM. The
> > above statement is antithetical to
> > Documentation/process/stable-api-nonsense.rst. If HMM is trying to set
> > expectations that device-driver projects can write to a "stable" HMM
> > api then HMM is setting those device-drivers up for failure.
>
> So i am not expressing myself correctly. If someone want to change mm
> in anyway that would affect HMM user, it can and it is welcome too
> (assuming that those change are wanted by the community and motivated
> for good reasons). Here by understanding HMM contract and preserving it
> what i mean is that all you have to do is update the HMM API in anyway
> that deliver the same result to the device driver. So what i means is
> that instead of having to understand each device driver. For instance
> you have HMM provide X so that driver can do Y; then what can be Z a
> replacement for X that allow driver to do Y. The point here is that
> HMM define what Y is and provide X for current kernel mm code. If X
> ever need to change so that core mm can evolve than you can and are
> more than welcome to do it. With HMM Y is defined and you only need to
> figure out how to achieve the same end result for the device driver.
>
> The point is that you do not have to go read each device driver to
> figure out Y.driver_foo, Y.driver_bar, ... you only have HMM that
> define what Y means and is ie this what device driver are trying to
> do.
>
> Obviously here i assume that we do not want to regress features ie
> we want to keep device driver features intact when we modify anything.

The specific concern is HMM attempting to expand the regression
boundary beyond drivers that exist in the kernel. The regression
contract that has priority is the one established for in-tree users.
If an in-tree change to mm semantics is fine for in-tree mm users, but
breaks out of tree users the question to those out of tree users is
"why isn't your use case upstream?". HMM is not that use case in and
of itself.

[..]
> Again HMM API can evolve, i am happy to help with any such change, given
> it provides benefit to either mm or device driver (ie changing the HMM
> just for the sake of changing the HMM API would not make much sense to
> me).
>
> So if after converting driver A, B and C we see that it would be nicer
> to change HMM in someway then i will definitly do that and this patchset
> is a testimony of that. Converting ODP to use HMM is easier after this
> patchset and this patchset changes the HMM API. I will be updating the
> nouveau driver to the new API and use the new API for the other driver
> patchset i am working on.
>
> If i bump again into something that would be better done any differently
> i will definitly change the HMM API and update all upstream driver
> accordingly.
>
> I am a strong believer in full freedom for internal kernel API changes
> and my intention have always been to help and facilitate such process.
> I am sorry this was unclear to any body :( and i am hopping that this
> email make my intention clear.''

A simple way to ensure that out-of-tree consumers don't come beat us
up over a backwards incompatible HMM change is to mark all the exports
with _GPL. I'm not requiring that, the devm_memremap_pages() fight was
hard enough, but the pace of new exports vs arrival of consumers for
those exports has me worried that this arrangement will fall over at
some point.

Another way to help allay these worries is commit to no new exports
without in-tree users. In general, that should go without saying for
any core changes for new or future hardware.